Wrong for YOU to want this? Yes. You aren't his parent even if you act in a parental role. It takes a lot of cajones to tell a mother that you, as a SM, are uncomfortable with how she is spending her visitation with her son.

Wrong for your SO to want this? No, he should be able to talk to his son at least once a week while he is away for 6 weeks. 

What is unreasonable in all of this is wondering what BM is doing with SS. If you have a valid reason to believe he is in danger (and I mean actual danger - exposure to unlocked and loaded firearms, being babysat by a sex offender with offenses against children, that kind of danger), then call the authorities for them to investigate. However, being an inconsistent parent isn't a crime, and she should be able to spend her 6 weeks with her son without interference from her ex and his SO.

If she wants SS to spend the majority of his time with her with his maternal family, so be it. My guess is that she can't take off a full 6 weeks, so he's staying with a babysitter or going to camps or visiting family so she can work. She shouldn't be villainized for that.

I understand wanting to talk to SS, but there is ZERO reason to harrass his mother when she doesn't have him call back. Unless there is a court order saying SS HAS to be available to talk, your SO has no right to demand to speak with his son, and you especially don't have a right to. Yeah, BM has to be trusted because she has rights to her kid that a court hasn't taken away (and likely won't unless she is actively abusing him, which it doesn't sound like she is).

If you are angry that I'm telling you that you need to take a seat and stop bothering BM for phone calls and questioning her intentions with her own son because you act as the primary parent, then you need to re-evaluate your status as primary parent. Talk to your SO about taking on less with SS. Even if you want to retain your primary parent status, you need to view yourself more in the role of a foster parent than a bio parent. Your job in that role is to support SS in having healthy relationships with his bio parents because that is going to be what helps him be his best in the long run. You can't replace BM; your SO made the decision that she is mom, for better or worse. What you can do is be loving, supportive, and strong for SS. However, you also have to let his bio parents do as they see fit, whether you like it or not, unless a court says otherwise.

So sit back, decompress, and trust that SS is safe with his BM, unless you have evidence otherwise and then involve CPS or the cops. Leave SS and BM alone.

It is really hard to let go of that control when you are the "mom" day in and day out and then expected to sit down and shut up because BM decided to show up. 

As much as it sucks, we can't care more than the bio parents do. We have no rights, we have no control that our spouse doesn't give us.

Your DH needs to be the one getting the updates and at least starting the calls with your SS. He has to be the driving force for communicating with BM, not you. I am so sorry you are dealing with the heartbreak of all of this mess and dealing with BM who is just living her life and leaving you to clean up the mess when she dances away again.